1. Field of the Invention
This invention relates to an advanced process for producing glycerol alkyl ethers with low amount of mono-alkyl ether by reacting glycerol and straight, branched or cyclic olefinic hydrocarbons, and/or the corresponding aldehydes, ketones and alcohols, in the presence of a homogeneous acid catalyst which enables the production of glycerol ethers free from by-products formed by olefin oligomerization.
2. Description of the Related Art
In recent years environmental restrictions suggested the application of biofuels as transportation fuel. Biodiesel has been implemented in many countries as a compulsory component of traditional oil based diesel fuel and the production of biodiesel from renewable sources is increasing rapidly. In the biodiesel process, besides biodiesel, derived from natural oils and fats by transesterification with lower alcohols as methanol or ethanol, also roughly 10% of glycerol in the form of a (watery) solution is produced as a by-product. The increase in biodiesel production resulted in an excess of glycerol on the chemical market and disturbed world market glycerol prices. One of the possible solutions for glycerol overproduction is the transformation into glycerol alkyl ethers, particularly into glycerol tert-butyl ethers which are exploitable as a diesel and biodiesel fuel additive.
The preparation of alkyl ethers of glycerol is a known reaction. Alkyl ethers of glycerol can be formed by the Williamson's synthesis (See U.S. Pat. No. 2,255,916) when alcoxide is added to a suitable alkyl halide to form the ether, or can be formed by the reaction of alcohol or olefin in the presence of an acid catalyst. Many patents concerning the glycerol alkyl ethers production via the etherification reaction of glycerol and olefin have been published. One of the first patents applied in this field is U.S. Pat. No. 1,968,033 (published in 1934) describing the tertiary ether formation from glycerol and isobutylene with the aid of sulfuric acid as homogeneous catalyst.
In general, the procedure is carried out by reacting glycerol and isobutylene using an acid catalyst. As described in patent U.S. Pat. No. 5,476,971 one of the methods for producing ditertiary butyl ether of glycerol is the reaction of glycerol and isobutylene in a two phase reaction system. The reaction mixture is phase separated into a heavier glycerol and catalyst containing polar phase and a lighter hydrocarbon phase from which product ethers can be readily separated. As a homogeneous catalyst, p-toluenesulfonic or methanesulfonic acid was used. Another conventional process (US 2007/0238905 A1) describes the process for converting glycerol to alkyl glycerol ethers from glycerol, alkyl alcohol and an etherification catalyst to obtain a reaction product comprising of alkyl glycerol ethers.
The alkyl ethers of glycerol are excellent oxygen additives for diesel and biodiesel fuel. The di- and tri-alkyl ethers show good miscibility with commercial diesel fuel and thus can be used in the concentrations required for the desired emissions reduction. As stated in U.S. Pat. No. 6,015,440 glycerol tert-butyl ethers added to biodiesel fuel proved the decrease in cloud-point and improved viscosity properties of biodiesel fuel mixed with glycerol tert-butyl ethers. According to patent U.S. Pat. No. 5,308,365 the use of ether derivatives of glycerol which, when incorporated in standard 30-40% aromatic containing diesel fuel, provides reduced emissions of particulate matter, hydrocarbons, carbon monoxide and unregulated aldehyde emission. The international application WO 2007/061903 A1 relates to compositions which include an alcohol and mixture of glycerol ethers, potentially derived from renewable sources. When combined with gasoline/ethanol blends, the glycerol ethers can reduce the vapour pressure of ethanol and increasing the fuel economy. When added to diesel fuel/alcohol blends, glycerol ethers improve the cetane value of the blends. Patent U.S. Pat. No. 5,578,090 describes a fuel additive composition including fatty acid alkyl esters and glyceryl ethers prepared by etherifying glycerol with one or more olefins in the presence of acid catalyst. Utilization of crude glycerol from the transesterification process is stated in patent US 2007/0283619 A1. In the transesterification process a heterogeneous catalyst was applied. The glycerol by-product from this process is free from catalyst and is at least 98% pure. It contains no metals, no neutralization salts and no additional purification is required. The glycerol obtained may be used directly in an etherification reaction with isobutylene in the presence of an acid catalyst.
Various typical properties of cited patent applications are listed below:    U.S. Pat. No. 2,255,916 by Doelling relates to the ethers of glycerol produced by Williamson's synthesis.    U.S. Pat. No. 1,968,033 by Evans teaches the reaction for preparing tertiary ethers of glycerol by using glycerol and isobutylene in the presence of sulfuric acid.    U.S. Pat. No. 5,476,971 by Gupta describes the process for preparation of ditertiary butyl ether preparation in the two separate liquid phases comprised from glycerol and isobutylene using homogeneous catalyst.    US 2007/0238905 by Arredondo patents the way of producing alkyl glycerol ethers by reacting glycerol with correspondent alkyl alcohol.    U.S. Pat. No. 6,015,440 by Noureddini concerns about the improved biodiesel composition comprised of methyl esters and glycerol ethers produced from the purified glycerol by-product formed in the transesterification process.    U.S. Pat. No. 5,308,365 by Kesling describes the positive influence of dialkyl and trialkyl derivatives of glycerol on particulate matter emissions when incorporated in conventional diesel fuel.    WO 2007/061903 by Bradin relates to compositions which include a mixture of glycerol ethers, which when combined with gasoline/ethanol blends, can reduce the vapour pressure of the ethanol.    U.S. Pat. No. 5,578,090 by Bradin deals with the fuel composition that includes fatty acids alkyl esters and glyceryl ethers, prepared by etherifying glycerol with one or more olefins in the presence of acid catalyst.    US 2007/0283619 by Hill provides a process where non-treated glycerol by-product (from triglycerides transformation) is reacted with an olefinic hydrocarbon to form glycerol ethers.